Monofilament produced by extruding thermoplastic from the nozzles of a spinning bar is usually stretched to improve the properties and decrease the fineness of the filamenting product. The spun monofilament passes through a blowing compartment, and a stretching chamber.
The spinning bar has a plurality of spinning orifices, from which the individual fibers emerge in a plastic state.
The blowing chamber is provided with blowing orifices through which air is fed as a cooling and stretching means.
The stretching chamber has stretching chamber walls with an accelerating constriction having a venturi nozzle-shape as seen in vertical cross section. The apparatus is generally transverse to the direction of flow of air and of the monofilament. In the scope of this invention, however, it is also possible for the stretching chamber cross section to be round, for example, generally circular.
In the known air flow stretching apparatus the stretching chamber walls comprise comparatively thick panels or sheets. They are not deformable when mounted in the completely assembled device. The venturi nozzle accelerating constriction is constructed with predetermined shape, and cannot be altered during operation.
When on account of the material which is used for the fibers or threads, on account of the particular degree of stretching desired, or for other reasons an alternative shape of the venturi like accelerating constriction of the stretching chamber walls is required, a new stretching chamber must be constructed, and substituted for the stretching chamber currently in the apparatus.
In operation, the cross section also cannot be changed for the purpose of correction or optimization of the stretching or spinning process. In the known stretching machines, the only variable operation parameters are the air flow rate and velocity. Unfortunately, completely satisfactory results cannot be obtained by controlling these parameters alone.